The nonlinear optical properties of the two oxazine dyes, i.e. oxazine 720 (OX720) and oxazine 750
(OX750), in aqueous solution and in polyacrylamide hydrogel (PAA) matrix were studied by Z-scan technique
using CW He–Ne laser at 632.8 nm. All the polymeric and aqueous samples showed a negative nonlinearity
and large nonlinear refractive index of the order of 108 cm2/W. The concentration-dependent
nonlinear refractive index was also investigated. In the case of PAA–dye systems, two types of transit
(fast) and steady state (slow) nonlinear effects were observed. It is believed that these effects are due
to the thermo-optical induced refractive index gradient and a concentration gradient induced by a temperature
gradient present in a medium.
